1. The Field of the Invention
The present invention relates to methods for preventing the fouling of boilers, furnaces, and other burner equipment during the noncatalytic reduction of nitric oxide ("NO") by ammonia ("NH.sub.3 ") or ammonia precursors. The invention, further, relates to methods for preventing the reaction of sulfur trioxide ("SO.sub.3) and ammonia downstream from the nitric oxide reaction zone to yield ammonium bisulfate ("NH.sub.4 HSO.sub.4 ") which precipitates from the gas phase and fouls the equipment which it contacts. More specifically, the present invention teaches methods for preventing NH.sub.4 HSO.sub.4 formation by the selective, noncatalytic, reduction of SO.sub.3 to SO.sub.2 with methanol, thereby making SO.sub.3 unavailable to react with NH.sub.3.
2. The Background of the Invention
One of the major problems in modern industrialized society is the production of air pollution from numerous sources. Air pollution can take various forms. Some of the different types of air pollutants include particulate emissions such as dust, coal particles and the like, sulfur compounds such as SO.sub.2 and SO.sub.3, ozone, carbon monoxide emissions, volatile hydrocarbon emissions, and the emission of nitrogen compounds commonly referred to collectively as "NO.sub.x ". Pollution sources include automobiles, industrial plants, small commercial establishments (such as dry cleaners and service stations), and even nature itself.
Combustion effluents and waste products from particular types of sources have proven to be major contributors to damaging air pollution when the effluents are discharged into the atmosphere. Unless these waste products are treated before their release into the atmosphere, serious smog and other air pollution problems are encountered. It will be readily appreciated that high concentrations of air pollutants can have serious deleterious impacts on the health and the general welfare of society. Air pollution is known to aggravate certain medical conditions (such as heart and lung problems) and is known to cause problems in the environment ranging from corrosion to acid rain.
One of the most common components found in polluted air is nitrogen dioxide ("NO.sub.2 "), which is known to be an extremely poisonous material. Nitrogen dioxide, which is brown in color, undergoes a series of reactions known generally as "photochemical smog formation" in the presence of sunlight and airborne hydrocarbons. These reactions result in a marked decline in overall air quality. While NO.sub.2 is produced from a wide variety of pollution sources, its primary source is from NO released into the air. NO is commonly formed during combustion processes, including internal combustion engines in automobiles, hydrocarbon fueled power plants, process furnaces, incinerators, coal fired utility boilers, glass furnaces, cement kilns, oil field steam generators, gas turbines, and other similar installations.
In these combustion processes, part of the ambient oxygen combines with atmospheric nitrogen (rather than with the fuel) in the flame by a process generally known as "nitrogen fixation." Nitrogen fixation is primarily limited to processes where a flame is employed in the combustion process, as opposed to catalytic processes. However, if there are organic nitrogen compounds in the fuel, they may also form NO when the fuel is burned either by a flame or catalytically.
Since NO is the only oxide of nitrogen which is stable at the high temperatures encountered in these types of combustion processes, NO is the predominant product. At normal atmospheric temperatures, however, the equilibrium between NO and NO.sub.2 favors NO.sub.2. Hence, NO formed by combustion is generally discharged into the atmosphere as NO, and only subsequently is converted to NO.sub.2. In order to control NO.sub.2 emissions, therefore, it is necessary to deal to a large extent with NO before it enters the ambient air.
There have been considerable efforts in the art to find effective ways to remove oxides of nitrogen from waste gases, so that these waste gases may be discharged to the atmosphere without harm to the environment. It has been found in the art that the removal of NO.sub.2 is relatively easy since it reacts with water and air to form nitric acid. NO.sub.2, therefore, is commonly removed by aqueous scrubbing.
If a base, such as ammonia, is added to the scrub water, the nitrogen scrubbing process is facilitated and ammonium nitrate is produced. If limited amounts of NO are present along with the NO.sub.2, the NO may be co-scrubbed, thereby yielding ammonium nitrate.
These processes, however, are subject to the limitation that they are only effective for mixtures of nitrogen oxides which are predominantly NO.sub.2, rather than predominantly NO. This is a problem because NO is the predominate species at the temperatures generally encountered in flue gases. As a result, various processes have been developed in the art for oxidizing NO to NO.sub.2 so that the relative inexpensive and convenient scrubbing processes may take place. For example, methods have been developed for oxidizing and removing NO.sub.x exhausted from boilers, cinder furnaces, nitric acid factories, automobiles, and the like.
Several processes known in the prior art involve contacting the gaseous flow which includes NO, with various organic compounds (such as aldehydes, alcohols, ketones, organic acids, and the like) in the presence of oxygen. By such processes, the NO is oxidized to NO.sub.2 which can then be removed by the scrubbing processes described above.
Similar processes have been developed whereby NO is oxidized to NO.sub.2 by contacting the NO with other types of oxygen-containing hydrocarbons such as methanol and ethanol.
An alternative approach for removing NO from flue gases and other streams of pollutants is to reduce NO to nitrogen and water, which may then be discharged to the atmosphere. Indeed, the present invention is expected to find its primary application in conjunction with such reduction processes. These processes generally teach the removal of NO.sub.x from flue gas by reduction of the NO by the addition of ammonia, or an ammonia precursor, alone or in combination with a second combustible material while the waste gas is at a relatively high temperature (generally from about 700.degree. C. to about 1200.degree. C.).
An example of such an NO reduction process is described in U.S. Pat. No. 3,900,554 to Lyon, issued Aug. 19, 1975 (hereinafter referred to as the "'554 patent"). The process disclosed in the '554 patent teaches the reduction of NO to N.sub.2 by injecting ammonia into the combustion effluent stream at a temperature from about 870.degree. C. to about 1100.degree. C. If the NH.sub.3 is injected with a second reducing agent, such as hydrogen, NO will be reduced to N.sub.2 at temperatures as low as 700.degree. C.
The method described in the '554 patent for controlling the emission of NO to the atmosphere has a number of advantages. The NO in the emission stream is converted to molecular nitrogen and water. This conversion is accomplished without the use of any catalyst and hence without the substantial expense and difficulties inherent in catalytic processing of combustion effluents.
Moreover, according to the process of the '554 patent, the conversion of NO to N.sub.2 is selective and specific. Combustion effluents typically contain NO at concentrations of from about 100 to about 2,000 ppm, while oxygen concentrations are generally in the range from about 2% to about 10%. Thus, the concentration of oxygen is orders of magnitude greater than the concentration of NO.
If the reduction of NO were nonselective (ie., if to reduce one percent of the NO, one had to reduce one percent of the oxygen), the amount of reducing agent required would be prohibitive. In the process described in the '554 patent, however, the reduction of NO is achieved with nearly equimolar quantities of NH.sub.3. The reaction is highly selective; reducing a large fraction of the NO while leaving the oxygen largely untouched.
Notwithstanding the foregoing advantages, the process described in the '554 patent, and other processes which employ a nitrogen oxide reduction technique, encounter a severe disadvantage in most applications. Many of the sources of the coal and oil commercially used in furnaces, boilers, and other burners, contain some amount of sulfur. In most instances, it is quite expensive and technically difficult to remove the sulfur before burning the fuel. It has been a common practice, therefore, to burn fuels which still contain a portion of the native sulfur. Sulfur compounds, including SO.sub.2 and SO.sub.3, are then produced during combustion and must be removed from the effluent gas stream by separate expensive technologies.
Combustion of a sulfur-containing fuel in a boiler, furnace, or other burner typically produces combustion effluents in which 98% to 99% of the sulfur exist in the form of SO.sub.2, and only 1% to 2% of the sulfur exists in the form of SO.sub.3. In processes such as that described in the '554 patent where NO is reduced by injecting ammonia into the gas stream, some of the ammonia or ammonia precursors injected into the process will be left unreacted. Under certain conditions NH.sub.3 will react with the sulfur gases, i.e., as the combustion effluents cool, the remaining NH.sub.3 will react with SO.sub.3 and water vapor present in the effluent stream to form NH.sub.4 HSO.sub.4 according to reaction equation (1): EQU NH.sub.3 +SO.sub.3 +H.sub.2 O.fwdarw.NH.sub.4 HSO.sub.4 ( 1)
Unfortunately, NH.sub.4 HSO.sub.4 is an extremely sticky and corrosive liquid and is known to damage the equipment used in combustion processes.
The temperature at which the formation of NH.sub.4 HSO.sub.4 occurs is such that, in a typical boiler or furnace of the type generally encountered in combustion processes, formation of NH.sub.4 HSO.sub.4 occurs within the air heater. Thus, fouling, corrosion, and plugging of the air heater has been commonly encountered. As a result, the nitric oxide reduction technologies which use ammonia and ammonia precursors have had severe limitations and their commercial acceptance has been correspondingly limited. The operators of boilers and furnaces are often extremely reluctant to accept any technology which can cause fouling, plugging, and corrosion such as that expected from NH.sub.4 HSO.sub.4.
It will be noted that in the event that there are no sulfur oxides in the effluent stream, the formation of NH.sub.4 HSO.sub.4 is not a problem. In many combustion applications, however, there will be at least a small quantity of sulfur oxides in the effluent stream and, thus, some SO.sub.3. The ammonia or ammonia precursors which are used in the nitric oxide reduction reactions can then react with the SO.sub.3 present to form NH.sub.4 HSO.sub.4, which even in small quantities can cause severe problems over time.
It is apparent that what is currently needed in the art is a method for the prevention of the formation of NH.sub.4 HSO.sub.4 during the reduction of NO.sub.x in the presence of ammonia, or ammonia precursors, when sulfur is present in the effluent stream. It would be an advancement in the art to provide such a method which was operable during the noncatalytic reduction of NO.
It would be a further advancement in the art to provide for the removal of SO.sub.3 from the effluent stream so that formation of NH.sub.4 HSO.sub.4 was prevented. It would be a further advancement in the art if SO.sub.3 could be selectively and noncatalytically reduced to SO.sub.2, which can be readily removed by conventional techniques.
Such methods and apparatus are disclosed and claimed below.